Controller of exhaust gas purifying agent and exhaust gas purifying system

ABSTRACT

A quantity of a purifying agent that is added into an exhaust gas, reacts with a specific component in the exhaust gas, and purifies the exhaust gas is controlled. A controller is configured so as to be provided with a program to carry out either increment control for increasing the quantity of urea water sequentially or decrement control for decreasing the quantity of the urea water sequentially while monitoring, in NOx in the exhaust gas, the quantity not purified through the reaction with NH 3  (the discharged NOx quantity); and switch the quantity control from the currently adopted increment or decrement control to the other increment or decrement control in response to whether the feature of the change of the discharged NOx quantity comes to a predetermined feature.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based on Japanese Patent Application No. 2007-46062 filed on Feb. 26, 2007, the disclosure of which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to an exhaust gas purifying agent controller to control a quantity of a purifying agent, for example ammonia (NH₃) added in the form of a urea aqueous solution or the like, to purify an exhaust gas by reacting with a specific component, for example nitrogen oxide (NOx) or the like, in the exhaust gas. The present invention also relates to an exhaust gas purifying system to purify an exhaust gas by activating exhaust gas purifying reaction with a predetermined purifying agent on a catalyst as represented by a urea Selective Catalytic Reduction (SCR) system.

BACKGROUND OF THE INVENTION

In power plants, various kinds of factories, automobiles, and others, a urea SCR system has been being developed. In the case of an automobile (an automobile on which a diesel engine is mounted in particular), as post-treatment technologies on an exhaust gas to purify and reduce NOx in the exhaust gas, the aforementioned urea SCR system or a NOx occlusion reduction catalyst is employed. The urea SCR system is practically used already for a large truck and known for a high purifying rate up to about “90%.” An ordinary urea SCR system that is currently studied for the practical application to a diesel engine is a system wherein NOx in an exhaust gas is reduced (purified) by NH₃ produced from a urea ((NH₂)2CO) aqueous solution (hereunder referred to as urea water). As a concrete example of such a urea SCR system, there is a known system configured so as to be provided with: an oxidation catalyst to oxidize parts of HC, CO, and PM in an exhaust gas; a Diesel Particulate Filter (DPF) to remove PM on the downstream of the oxidation catalyst; a urea water feed valve to add (supply by injection) urea water on the downstream of the DPF; an SCR catalyst to reduce NOx by NH₃ (a purifying agent) produced by hydrolyzing ((NH₂)2CO+H₂O→2NH₃+CO₂) urea water added through the urea water feed valve on the further downstream side; and an NH₃ catalyst, for example oxidation catalyst, to purify excessive NH₃ that is not consumed through the reduction reaction and flows on the downstream of the SCR catalyst and reduce the quantity of NH₃ discharged into the air.

In such a urea SCR system, the reduction of the quantity of NH₃ sent to an NH₃ catalyst as well as the improvement of a NOx purifying rate is important. This is because in general NH₃ is known as poisonous and also the strong odor adversely affects the environment. Then a technology important for obtaining both the improvement of a NOx purifying rate and the reduction of an NH₃ quantity is the precise control of the urea water quantity. When the quantity of urea water is insufficient in comparison with a NOx quantity (an unpurified NOx quantity) in an exhaust gas to be purified, NOx is purified insufficiently. In contrast, when the quantity of urea water is excessive in comparison with an unpurified NOx quantity, excessive NH₃ that has not been consumed through the reduction reaction with NOx is sent to the NH₃ catalyst. Consequently, it is desirable to control the quantity of urea water so that the purified NOx quantity (or the NOx purifying rate) may be the maximum and the redundant NH₃ quantity (the quantity of NH₃ not consumed through the reduction reaction with NOx) may be the minimum. The performance of an exhaust gas purifying system improves as the purifying rate of NOx to be purified increases. With regard to NH₃, it is important to configure a system producing a less excessive quantity of NH₃, namely a system allowing excessive (unreacted) NH₃ to be removed only with a small NH₃ catalyst or without the use of an aforementioned NH₃ catalyst if possible. With such a system, it is possible to save a space and reduce the cost. Further, it is also important to reduce the redundant NH₃ quantity from the viewpoint of securing the longer service life of an NH₃ catalyst.

A controller to control the quantity of urea water as described in JP-A-2003-269142 has been known for example. The controller estimates the quantity of unpurified NOx discharged from a diesel engine on the basis of a predetermined engine running state, for example, an engine speed, an intake air flow rate in a predetermined cylinder and the like. When the urea water is added, a urea water feed valve is controlled so as to add the urea water by a quantity necessary for completely purifying NOx corresponding to the estimated unpurified NOx quantity. This suppresses the excessive addition of urea water while the NOx purifying rate is maintained at a high level. Further, by the reduction of a redundant NH₃ quantity, the possibilities of realizing the longer service life and the simplification (downsizing) of an NH₃ catalyst also increase.

As stated above, the device described in JP-A-2003-269142 is configured so as to control the quantity of urea water to an appropriate quantity on the basis of the unpurified NOx quantity estimated from an engine running state. However, the engine running state includes many parameters that influence the unpurified NOx quantity and it is likely that the unpurified NOx quantity may change by a parameter other than the engine running state. Further, even when the unpurified NOx quantity does not change, it is likely that the purification performance (the purifying rate) of an SCR catalyst or an NH₃ catalyst varies in accordance with the change of the state of an exhaust gas and a catalyst and the optimum value of the quantity of urea water may change. For example, a device described in JP-A-2003-301737 controls the quantity of urea water on the basis of the temperature of an exhaust gas and the temperature of a catalyst. Further, in the device described in JP-A-2003-314256, a NOx sensor is installed on the downstream side of an SCR catalyst, an adsorption of NH₃ to the SCR catalyst is computed on the basis of an output of the sensor, and the quantity of urea water is controlled on the basis of the computed NH₃ adsorption.

However, it is not realistic to estimate an unpurified NOx quantity (or an optimum value of urea water quantity) on the basis of all those parameters because a huge memory capacity is required in order to store a multidimensional map (or a mathematical expression, etc.) or the computation quantity for the estimation of an unpurified NOx quantity increases. Consequently, even in the case of estimating an unpurified NOx quantity (or an optimum value of urea water quantity) from an engine running state, etc., it is difficult to obtain a sufficiently precise value. As stated above, even with a device described in JP-A-2003-269142, it is difficult to control the quantity of urea water (an additive) that is the source of a purifying agent (NH₃) to an optimum value with a sufficiently high degree of accuracy so that the purified NOx quantity (or a NOx purifying rate) may be the maximum and a redundant NH₃ quantity (the quantity of NH₃ not consumed through reduction reaction with NOx) or a discharged NH₃ quantity (the quantity of NH₃ not purified with an NH₃ catalyst) may be the minimum.

SUMMARY OF THE INVENTION

The present invention has been established in view of the above situation and an object of the present invention is to provide: a quantity controller of an exhaust gas purifying agent capable of flexibly responding to various situations and controlling the quantity of an exhaust gas purifying agent to an optimum quantity with a high degree of accuracy; and an exhaust gas purifying system capable of properly purifying an exhaust gas with the controller.

According to the present invention, a controller controls the quantity of a purifying agent added into an exhaust gas to purify the exhaust gas by reacting with a specific component in the exhaust gas. The controller is provided with a quantity control means for: carrying out first quantity control for increasing or decreasing the quantity of the purifying agent sequentially at a predetermined rate (a fixed value or a variable responding to a predetermined parameter is acceptable) while monitoring, in the specific component in the exhaust gas, the quantity of the unpurified specific component corresponding to the quantity not purified through the reaction with the purifying agent. When the feature of the change of the unpurified specific component quantity comes to a predetermined feature (a fixed feature or a variable feature responding to a predetermined parameter is acceptable) by the implementation of the first quantity control, the first quantity control is stopped and then, contrary to the first quantity control, carrying out second quantity control for decreasing or increasing the quantity of the purifying agent sequentially at a predetermined rate (a fixed value or a variable is acceptable) while monitoring the quantity of the unpurified specific component. When the feature of the change of the unpurified specific component quantity comes to a predetermined feature (a fixed or variable feature different from the feature in the first quantity control is acceptable) by the implementation of the second quantity control, the second quantity control is stopped.

According to another aspect of the present invention, a controller is provided with a practice control switching means for: carrying out either first quantity control for increasing the quantity of the purifying agent sequentially at a predetermined rate (a fixed value or a variable is acceptable) or second quantity control for decreasing the quantity of the purifying agent sequentially at a predetermined rate (a fixed value or a variable is acceptable) while monitoring, in the specific component in the exhaust gas, the quantity of the unpurified specific component corresponding to the quantity not purified through the reaction with the purifying agent. In response to whether or not the feature of the change of the unpurified specific component quantity comes to a predetermined feature (a fixed or variable feature is acceptable), switching the quantity control from the currently adopted first or second quantity control to the other first or second quantity control.

As stated above, the optimum value of the quantity of a purifying agent (the quantity of urea water described in the aforementioned controller) varies in accordance with many parameters. Consequently, it is difficult to obtain an optimum purifying agent quantity from those parameters. In view of the situation, the aforementioned controller controls the quantity of a purifying agent more directly to an optimum value.

That is, as stated above, the quantity of a purifying agent takes a so-called optimum value when the quantity (or the purifying rate) of a purified specific component, for example NOx, in the specific component to be purified is the maximum and the quantity of the redundant purifying agent (a redundant NH₃ quantity) or the quantity of the discharged purifying agent (a discharged NH₃ quantity) is the minimum. The minimum quantity required for making the quantity of a purified specific component maximum (that is, making the unpurified specific component quantity minimum) corresponds to the aforementioned optimum value, and the following results have been obtained with regard to the relationship between the quantity of a purifying agent and the quantity of a purified specific component. That is, when a purified quantity approaches the maximum value from the state of a low purified quantity level while the quantity is increased sequentially, the state of the change (the feature of the change) of the purified quantity before the purified quantity comes to the maximum is largely different from the state after the purified quantity comes to the maximum. Meanwhile, when a purified quantity is reduced by sequentially decreasing the quantity from the state where the purified quantity is the maximum, the state of the change (the feature of the change) of the purified quantity when the purified quantity is the maximum is largely different from the state where the purified quantity is lower than the maximum. With regard to a purified quantity, the quantity of a purifying agent at the turning point of the feature of the change corresponds to the minimum quantity required for making an unpurified specific component quantity minimum (the purified quantity is the maximum), namely the optimum value of the quantity. According to the present invention, even when the quantity of a purifying agent is in an excessive region or an insufficient region, it is possible to bring the quantity of a purifying agent close to an optimum value by either the first or second quantity control. Even when the quantity of the unpurified specific component itself is not confirmed, it is possible to control the quantity of a purifying agent to an optimum value by the feature of the change of an unpurified specific component quantity. As a result, even when the optimum value of the quantity varies due to aging or the like, the accuracy (convergence to an optimum value) of the quantity control can be maintained at a high level.

According to another aspect of the present invention, a controller controls the quantity of a purifying agent added into an exhaust gas to purify the exhaust gas by reacting with a specific component in the exhaust gas. The controller is provided with a quantity control means for: carrying out first quantity control for increasing or decreasing the quantity of the purifying agent sequentially at a predetermined rate (a fixed value or a variable is acceptable) while monitoring, in the purifying agent, the quantity of the unreacted purifying agent corresponding to the quantity not consumed through the reaction with the specific component. When the feature of the change of the unreacted purifying agent quantity comes to a predetermined feature (a fixed or variable feature is acceptable) by the implementation of the first quantity control, the first quantity control is stopped and then, contrary to the first quantity control, carrying out second quantity control for decreasing or increasing the quantity of the purifying agent sequentially at a predetermined rate (a fixed value or a variable is acceptable) while monitoring the quantity of the unreacted purifying agent. When the feature of the change of the unreacted purifying agent quantity comes to a predetermined feature (a fixed or variable feature is acceptable) by the implementation of the second quantity control, the second quantity control is stopped.

According to another aspect of the present invention, a controller is provided with a quantity control means for: carrying out first quantity control for increasing or decreasing the quantity of the purifying agent sequentially at a predetermined rate (a fixed value or a variable is acceptable) while monitoring, in the purifying agent, the quantity of the discharged purifying agent corresponding to the quantity discharged in the air on the downstream where the purifying agent reacts with the specific component. When the feature of the change of the unreacted purifying agent quantity comes to a predetermined feature (a fixed or variable feature is acceptable) by the implementation of the first quantity control, the first quantity control is stopped. Then, contrary to the first quantity control, the quantity control means carries out second quantity control for decreasing or increasing the quantity of the purifying agent sequentially at a predetermined rate (a fixed value or a variable is acceptable) while monitoring the quantity of the unreacted purifying agent. When the feature of the change of the unreacted purifying agent quantity comes to a predetermined feature (a fixed or variable feature is acceptable) by the implementation of the second quantity control, the second quantity control is stopped.

According to another aspect of the present invention, a controller is provided with a practice control switching means for: carrying out either first quantity control for increasing the quantity of the purifying agent sequentially at a predetermined rate (a fixed value or a variable is acceptable) or second quantity control for decreasing the quantity of the purifying agent sequentially, at a predetermined rate (a fixed value or a variable is acceptable) while monitoring, in the purifying agent, the quantity of the unreacted purifying agent corresponding to the quantity not consumed through the reaction with the specific component. In response to whether or not the feature of the change of the unreacted purifying agent quantity comes to a predetermined feature, the switching means switches the quantity control from the currently adopted first or second quantity control to the other first or second quantity control.

According to another aspect of the present invention, a controller is provided with a practice control switching means for: carrying out either first quantity control for increasing the quantity of the purifying agent sequentially at a predetermined rate (a fixed value or a variable is acceptable) or second quantity control for decreasing the quantity of the purifying agent sequentially at a predetermined rate (a fixed value or a variable is acceptable) while monitoring, in the purifying agent, the quantity of the discharged purifying agent corresponding to the quantity discharged in the air on the downstream where the purifying agent reacts with the specific component. In response to whether or not the feature of the change of the discharged purifying agent quantity comes to a predetermined feature, the switching means switches the quantity control from the currently adopted first or second quantity control to the other first or second quantity control.

By adopting either the first or second quantity control, it is possible to bring the quantity of a purifying agent close to an optimum value from an excessive region or an insufficient region. As already mentioned, accuracy in quantity control (convergence to an optimum value) can be maintained at a high level even when aging or the like exists.

The aspect (trend) of the variation of an unreacted purifying agent quantity or a discharged purifying agent quantity against the change of the quantity is opposite the aspect (trend) of the variation of an unpurified specific component quantity. Consequently, it is effective to configure that either the first quantity control or the second quantity control is the control to decrease the quantity of the purifying agent sequentially at a predetermined rate and to stop when the quantity of an unreacted purifying agent exceeds an allowable range and does not change any more even when the quantity is decreased; and the other quantity control is the control to increase the quantity of the purifying agent sequentially at a predetermined rate and to stop when the quantity of the unpurified specific component exceeds an allowable range and changes by the increase of the quantity. It is possible to control a quantity easily in an appropriate manner.

According to another aspect of the present invention, a controller is provided with a quantity control means for: carrying out quantity control for increasing or decreasing the quantity of the purifying agent sequentially from a predetermined initial value at a predetermined rate while monitoring, in the specific component in the exhaust gas, the quantity of the unpurified specific component corresponding to the quantity not purified through the reaction with the purifying agent. When the feature of the change of the unpurified specific component quantity comes to a predetermined feature by the implementation of the quantity control, the quantity control is stopped.

According to another aspect of the present invention, a controller is provided with a quantity control means for: carrying out quantity control for increasing or decreasing the quantity of the purifying agent sequentially from a predetermined initial value at a predetermined rate while monitoring, in the purifying agent, the quantity of the unreacted purifying agent corresponding to the quantity not consumed through the reaction with the specific component. When the feature of the change of the unpurified specific component quantity comes to a predetermined feature by the implementation of the quantity control, the quantity control is stopped.

According to another aspect of the present invention, a controller is provided with a quantity control means for: carrying out quantity control for increasing or decreasing the quantity of the purifying agent sequentially from a predetermined initial value at a predetermined rate while monitoring, in the purifying agent, the quantity of the discharged purifying agent corresponding to the quantity discharged in the air on the downstream where the purifying agent reacts with the specific component. When the feature of the change of the unpurified specific component quantity comes to a predetermined feature by the implementation of the quantity control, the quantity control is stopped.

It is also possible to: monitor any one of an unpurified specific component quantity, an unreacted purifying agent quantity, and a discharged purifying agent quantity; and derive or detect the optimum value of the purifying agent quantity from the track or the like (detected by plotting in a graph for example).

According to another aspect of the present invention, a controller is provided with a means for detecting the quantity of the purifying agent with which the quantity of the unpurified specific component is the minimum and also the quantity of the unreacted purifying agent is the minimum by monitoring at least one of, in the specific component in the exhaust gas, the quantity of the unpurified specific component corresponding to the quantity not purified through the reaction with the purifying agent and, in the purifying agent, the quantity of the unreacted purifying agent corresponding to the quantity not consumed through the reaction with the specific component.

According to another aspect of the present invention, a controller is provided with a means for detecting the quantity of the purifying agent with which the quantity of the unpurified specific component is the minimum and also the quantity of the discharged purifying agent is the minimum by monitoring at least one of, in the specific component in the exhaust gas, the quantity of the unpurified specific component corresponding to the quantity not purified through the reaction with the purifying agent and, in the purifying agent, the quantity of the discharged purifying agent corresponding to the quantity discharged in the air on the downstream where the purifying agent reacts with the specific component.

It is possible, on the basis of each detection value to carry out data analysis and failure diagnosis, and to control a purifying agent quantity to an optimum value with a high degree of accuracy.

When the practicability of the above invention is considered in view of the demand from society (public interest), as represented by the aforementioned urea SCR (Selective Catalytic Reduction) system, a NOx purifying device that uses a urea aqueous solution (urea water) as an additive is expected as an exhaust gas purifying system to purify NOx (nitrogen oxide) in an exhaust gas at a high purifying rate. The reason why NH₃ is added in the form of a urea aqueous solution is that a urea aqueous solution is easier to handle (less poisonous) than NH₃. Consequently, considering the practicability of the current situation, it is effective to configure a controller such that the specific component is NOx and the purifying agent is NH₃ produced by being added in the form of a urea aqueous solution and decomposing in an exhaust gas. Further, when such a urea SCR system is adopted in an automobile and a controller is mounted on a vehicle or the like having a diesel engine for example, it is possible to realize the improvement of fuel consumption and the reduction of PM while allowing NOx to be generated during combustion. This is a technology that largely contributes to the improvement in the performance of an automobile and the cleaning of an exhaust gas that is a key to the prevalence of diesel engines.

In some business sectors, applications, and others, not such a unit of a controller but a larger unit is sometimes used as an exhaust gas purifying system comprising various devices including not only such a controller but also other related devices (for example, various kinds of devices related to control such as sensors and actuators). That is, an exhaust gas purifying system is characterized by being provided with: a catalyst to accelerate specific exhaust gas purifying reaction; a purifying agent feed valve to add a purifying agent to purify an exhaust gas by carrying out the exhaust gas purifying reaction with a specific component in the exhaust gas on the catalyst or an additive acting as the source of the purifying agent to the catalyst itself or the exhaust gas on the upstream side of the catalyst; and a practice control switching means for carrying out either first quantity control for increasing the quantity of the purifying agent sequentially at a predetermined rate (a fixed value or a variable is acceptable) or second quantity control for decreasing the quantity of the purifying agent sequentially at a predetermined rate (a fixed value or a variable is acceptable) while monitoring, in the specific component in the exhaust gas, the quantity of the unpurified specific component corresponding to the quantity not purified through the exhaust gas purifying reaction, and, in response to whether or not the feature of the change of the unpurified specific component quantity comes to a predetermined feature (a fixed or variable feature is acceptable), switching the quantity control from the currently adopted first or second quantity control to the other first or second quantity control. Further, in this case, it is effective to use the urea SCR system, wherein the specific component is NOx (nitrogen oxide), the additive is a urea aqueous solution, and the catalyst accelerates NOx reduction reaction to reduce NOx through NH₃ (ammonia) produced by decomposing the urea aqueous solution as the exhaust gas purifying reaction. The same is applied also to other cases and it is particularly effective to use a controller according to the present invention by incorporating the controller into an exhaust gas purifying system (a urea SCR system in particular).

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings, in which:

FIG. 1 is a schematic view showing a quantity controller and an exhaust gas purifying system according to an embodiment of the present invention;

FIG. 2 is a flowchart showing a procedure of increment processing in a quantity control of urea water;

FIG. 3 is a flowchart showing a procedure of decrement processing in the quantity control of urea water; and

FIG. 4 is a graph showing the relationship of a quantity of urea water with a discharged NOx quantity and a discharged NH₃ quantity, respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present invention is hereunder explained in reference to drawings. The basic configuration of the exhaust gas purifying system in the embodiment conforms to that of an ordinary urea Selective Catalytic Reduction (SCR) system that has been stated earlier. A shown in FIG. 1, NOx in an exhaust gas is reduced (purified) by NH₃ (ammonia) produced from a urea ((NH₂)₂CO) aqueous solution (hereunder referred to as urea water).

The configuration of the exhaust gas purifying system is described in detail in reference to FIG. 1.

FIG. 1 is a configuration diagram showing the outline of a urea SCR system (an exhaust gas purifying device) according to the present embodiment.

As shown in FIG. 1, such a system purifies an exhaust gas discharged from a reciprocal diesel engine (an exhaust gas source) mounted on a four-wheeled automobile (not shown in the figure) for example. The system includes various kinds of actuators to purify the exhaust gas, various kinds of sensors, and an Electronic Control Unit (ECU) 40.

An exhaust gas purifying system is provided with a Diesel Particulate Filter (DPF) 11, an exhaust pipe 12 (an exhaust gas channel), an SCR catalyst 13, an exhaust pipe 14 (an exhaust gas channel), an NH₃ catalyst 15 (for example oxidation catalyst) in this order along an exhaust gas flow in the pipe 12. A urea water feed valve 16 is installed on the channel wall in the middle of the exhaust pipe 12 to add (supply by injection) urea water pumped from a urea water tank 17 a to the downstream side of the DPF 11 in the manner of directing a nozzle 16 a toward the exhaust gas downstream side so as not to contaminate the nozzle 16 a. In the present embodiment, the urea water feed valve 16 is configured so that the drive thereof is electrically controlled with the ECU 40, namely as an electromagnetically driven injection valve. Then by controlling the urea water feed valve 16 to a desired situation with the ECU 40, a desired quantity of urea water as an additive is supplied by injection to the exhaust gas flowing in the exhaust pipe 12 located between the DPF 11 and the SCR catalyst 13. The added urea water (or NH₃ after decomposed) as well as the exhaust gas is supplied to the SCR catalyst 13 on the downstream side by using the flow of the exhaust gas (exhaust gas flow).

That is, in the system, urea water is added through the urea water feed valve 16 and thereby NH₃ (a purifying agent) is produced from the urea water in an exhaust gas through decomposition reaction (Expression 1) described below. Then NOx reduction reaction (Expression 2) with NH₃ described below occurs on the SCR catalyst 13 and thereby the exhaust gas to be purified is purified (NOx purification).

(NH₂)2CO+H₂O→2NH₃+CO₂  (Expression 1)

NO+NO₂+2NH₃→2N₂+3H₂O  (Expression 2)

Then, excessive NH₃ (redundant NH₃) that is not consumed through the reduction reaction (Expression 2) and flows to the downstream side of the SCR catalyst 13 (the exhaust pipe 14) is purified through the reaction described below (Expression 3) with the NH₃ catalyst 15 and the quantity of NH₃ discharged into the air is reduced. Here, with regard to the exhaust gas on the downstream side of the SCR catalyst 13, the NH₃ quantity (the redundant NH₃ quantity) and the NOx quantity (the discharged NOx quantity) in the exhaust gas can be detected (the details are computed with the ECU 40 on the basis of the output from each sensor) with an exhaust gas sensor 14 a (a sensor including a NOx sensor and an NH₃ sensor) disposed in the exhaust pipe 14.

4NH₃+3O₂→2N₂+6H₂O  (Expression 3)

Successively, the various kinds of the exhaust gas purifying devices constituting the exhaust gas purifying system of the present embodiment are explained in detail.

The DPF 11 is a continuously regenerative type filter to capture and remove Particulate Matter (PM) in an exhaust gas and can be used continuously, for example, by repeatedly combusting and removing the captured PM at the post-injection after the main fuel injection or the like (corresponding to regeneration processing). The DPF 11 supports a platinum system oxidation catalyst (not shown). The oxidation catalyst may be installed as an independent unit. It is possible to remove HC and CO together with a soluble organic fraction (SOF) that is one of the PM components and oxidize a part of NOx into NO₂ (the purifying rate of NOx increases as the ratio of NO to NO₂ approaches one from Expression 2).

The SCR catalyst 13 is formed, for example, by making a catalyst carrier having a honeycomb structure support a catalyst metal made of, for example, vanadium oxide (V2O5) or the like. By the catalytic action, the reduction reaction (the exhaust gas purifying reaction) of NOx, namely above Expression 2, is accelerated.

The structure of the urea water feed valve 16 conforms to that of an ordinary fuel injection valve (an injector) used for supplying fuel to an engine (an internal combustion engine) for an automobile. Since the ordinary fuel injection valve is publicly known, the structure of the urea water feed valve 16 is explained just briefly. The urea water feed valve 16 includes a needle drive section comprising an electromagnetic solenoid and others; and a needle that is driven by the needle drive section, reciprocally moves in the valve body (the housing), and opens and closes necessary number of nozzle holes themselves pierced at the nozzle 16 a at the tip of the valve body or flow channels up to the nozzle holes. In the urea water feed valve 16, when the electromagnetic solenoid is electrified in accordance with an electric signal, for example, a pulsed signal in Pulse Width Modulation (PWM) control, from the ECU 40 on the basis of the structure (elements), namely in accordance with an injection command from the ECU 40, the needle is driven by the electromagnetic solenoid and moves toward the opening of the valve. Thereby the nozzle 16 a at the tip of the valve body opens, namely at least one of the nozzle holes of the nozzle 16 a opens, and the urea water is added (injected) into the exhaust gas flowing in the exhaust pipe 12. The quantity (the injection quantity) of the urea water is determined on the basis of the time duration when the electromagnetic solenoid is electrically activated, for example, corresponding to a pulse width of a pulsed signal from the ECU 40.

A urea water supply system to pump and supply urea water to the urea water feed valve 16 roughly comprises a urea water tank 17 a and a pump 17 b. The urea water stored in the urea water tank 17 a is pumped up with the pump 17 b installed in the tank 17 a and pumped toward the urea water feed valve 16. Then the pumped urea water is sequentially supplied to the urea water feed valve 16 through a pipe 17 c for urea water supply.

Foreign matters in the urea water are removed with a filter 17 f disposed on the upstream side of the urea water feed valve 16 before the urea water is supplied to the urea water feed valve 16. The pressure for supplying the urea water to the feed valve 16 is adjusted with a urea water pressure regulator 17 d. When the feed pressure exceeds a predetermined value, the urea water in the pipe 17 c returns into the urea water tank 17 a with a mechanical device using a spring or the like. In the present system, the feed pressure of the urea water is controlled to a predetermined value (a set pressure) by the action of the regulator 17 d. However, the feed pressure of the urea water is not always precisely controlled to a set pressure even by the action of the regulator 17 d and hence the present system is configured so that the feed pressure of the urea water may be detected (more specifically, computed in the ECU 40 on the basis of a sensor output) with a urea water pressure sensor 17 e disposed at a predetermined detection spot, for example, on the downstream side of the regulator 17 d where the fuel pressure is adjusted and stabilized with the regulator 17 d.

In such a system, a section that proactively carries out control related to exhaust gas purification as an electronic control unit is the ECU 40 (for example, an ECU for exhaust gas purification control connected to an ECU for engine control through a CAN or the like). The ECU 40 comprises a known microcomputer (not shown) and carries out various kinds of control related to the exhaust gas purification in an optimum feature responding to the current situation by operating various actuators such as the urea water feed valve 16 and others on the basis of detection signals from the various sensors. The microcomputer mounted on the ECU 40 basically comprises various kinds of arithmetical units, memories, signal processors, communication systems, power supply circuits, and others such as a central processing unit (CPU) to carry out various kinds of computation, a Random Access Memory (RAM) as a main memory to temporarily store the data under computation and computation results, a Read Only Memory (ROM) as a program memory, an Electronically Erasable & Programmable Read Only Memory (EEPROM) as a memory for data storage, a backup RAM (a RAM electrically fed from a backup electric power source such as an in-vehicle battery), signal processors including an A/D converter, a clock generation circuit, and others, and I/O ports to input and output signals from the exterior. Then beforehand a program related to the quantity control of an exhaust gas purifying agent, various programs and control maps related to exhaust gas purifying control are stored in the ROM and various control data and others including engine design data are stored in the memory for data storage, for example the EEPROM.

The configuration of an exhaust gas purifying system according to the present embodiment has been described above in detail. That is, in the present embodiment, the system is configured in the aforementioned feature so that NH₃ as a purifying agent is added in the form of a urea aqueous solution (urea water) into an exhaust gas with the urea water feed valve 16. This produces the urea water decomposes and NH₃ in the exhaust gas. Then NOx reduction reaction (Expression 2) is carried out on the SCR catalyst 13 with NH₃ and the exhaust gas to be purified (the exhaust gas from an engine) is purified. Moreover in the present embodiment, by applying the processing shown FIG. 2 and the processing shown in FIG. 3 as the quantity control of the urea water, it is possible to flexibly respond to more kinds of situations and control the quantity of the exhaust gas purifying agent, namely the quantity of the urea water, to an appropriate value with a high degree of accuracy. The quantity control of urea water is explained hereunder in reference to FIGS. 2 and 3.

FIGS. 2 and 3 are flowcharts showing the procedures of the processing related to the quantity control of urea water respectively. Basically, the series of processing shown in the figures is repeatedly carried out steadily at predetermined processing intervals (for example, from the start to the stop of an engine) by carrying out the program stored in the ROM at the ECU 40. The processing shown in FIG. 2 and the processing shown in FIG. 3 are carried out alternately. An increment flag and a decrement flag show the execution conditions in the processing. The values of various parameters used in the processing are occasionally stored, for example, in the memories such as a RAM, an EEPROM, and others mounted on the ECU 40 and renewed occasionally if necessary.

The series of processing shown in FIG. 2 is related to the control to increase the quantity of urea water sequentially at a predetermined rate (first quantity control). At Step S11, it is determined whether or not “1” is set at an increment flag showing whether the increment control of the urea water quantity should be carried out. Then when it is determined that “1” is set at the increment flag at Step S11, the increment control is considered to be necessary and the processing advances to Step S12 and the succeeding steps. When it is determined that “1” is not set at the increment flag at Step S11, the increment control is considered to be unnecessary, the processing is terminated as it is, and the processing at Step S12 and the succeeding steps is not carried out.

At Step S12, the urea water quantity Q is increased by a predetermined quantity a that is a fixed value or a variable value responding to an engine running state or the like.

Q=Q(previous value)+α

The urea water quantity Q is a parameter to decide the quantity of the urea water fed with the urea water feed valve 16, that is, a command value (an electrification pulse) to the feed valve 16. The quantity Q is stored in an EEPROM (or a backup RAM or the like) in the ECU 40. Even when the ECU 40 is once turned off at the time of engine stop and reactivated again, the data stored there are not erased and retained. As the initial value of the urea water quantity Q, an appropriate value stipulated by a predetermined map or the like is set.

At succeeding Step S13, it is determined whether or not the discharged NOx quantity has changed (increased or decreased) beyond a predetermined range (a fixed value, or a variable value responding to an engine running state). For example, it is determined whether or not the difference between the discharged NOx quantity before the urea water quantity Q is increased and the discharged NOx quantity after the increment is larger than a predetermined value. The discharged NOx quantity corresponds to the quantity (the unpurified specific component quantity) not purified by the reaction with NH₃ (reaction on an SCR catalyst 13) in NOx in an exhaust gas discharged from an engine (an exhaust gas source). The discharged NOx is measured, for example, with an exhaust gas sensor 14 a. Then during the time when it is determined that the discharged NOx quantity has changed at Step S13, the processing of Steps S11 and S12 is repeated at predetermined processing intervals and the urea water quantity Q increases sequentially by a predetermined rate α.

When it is determined that the discharged NOx quantity has not changed at Step S13, “0” is set at the increment flag and “1” is set at the decrement flag showing whether or not the decrement control of the urea water quantity is applied at succeeding Step S14. This determines that “1” is not set at the increment flag at preceding Step S11 and processing of Step S12 and the succeeding steps is not applied. That is, the increment control stops when the discharged NOx quantity does not change beyond an allowable range even though the urea water quantity Q is increased.

In the processing shown in FIG. 3, at Step S21, whether or not “1” is set at a decrement flag is determined and, only when it is determined that “1” is set at the decrement flag at Step S21, the processing at Step S22 and the succeeding steps is carried out.

At Step S22, the urea water quantity Q (a value renewed in the processing shown in FIG. 2 is continually used) decreases (loses weight) by a predetermined quantity β that is a fixed value, or a variable value responding to an engine running state.

Q=Q(previous value)−β

At succeeding Step S23, it is determined whether or not the discharged NOx quantity has changed (increased or decreased) beyond a predetermined allowable range that is a fixed value, or a variable value responding to an engine running state. For example, it is determined whether or not the difference between the discharged NOx quantity before the urea water quantity Q is decreased and the discharged NOx quantity after the decrement is larger than a predetermined value. Then during the time when it is determined that the discharged NOx quantity has not changed at Step S23, the processing of Steps S21 and S22 is repeated at predetermined processing intervals and thereby the urea water quantity Q decreases sequentially at a predetermined rate β.

When it is determined that the discharged NOx quantity has changed at Step S23, “1” is set at the increment flag and “0” is set at the decrement flag at succeeding Step S24. Further, the urea water quantity Q at the time is stored in an EEPROM (or a backup RAM or the like) in the ECU 40 in the state of being linked with the acquisition time (for example, the present date and hour). The stored urea water quantity Q corresponds to the optimum value of the urea water quantity at the time, namely a value where the purified NOx quantity is the maximum and the redundant NH₃ quantity (the quantity of NH₃ sent to an NH₃ catalyst 15) is the minimum. It is determined that “1” is not set at the decrement flag at the preceding Step S21, and the processing of Step S22 and the succeeding steps is not applied. That is, the decrement control stops when the discharged NOx quantity changes beyond an allowable range by decreasing the urea water quantity Q.

Next, an embodiment of the quantity control of urea water with the ECU 40 is explained briefly in reference also to FIG. 4. FIG. 4 is a graph showing the relationship of a quantity of urea water (an quantity injected from the urea water feed valve 16) on the horizontal axis with a discharged NOx quantity (L1) and a discharged NH₃ quantity (L2) measured with an exhaust gas sensor 14 a, respectively. The graph schematically shows the results of experiments and others conducted by the present inventors. The discharged NH₃ quantity corresponds to the quantity of NH₃ not consumed through reaction with NOx (the unreacted purifying agent quantity) in the NH₃ produced by decomposing urea water in an exhaust gas, namely the quantity of NH₃ sent to an NH₃ catalyst 15.

As shown with the solid line L1 in FIG. 4, in the region R1 where the urea water quantity is sufficiently low, as the quantity of urea water increases, the discharged NOx quantity (the slid line L1) decreases and. In the region R2 where the urea water quantity is larger than the optimum value of the urea water quantity (the optimum urea water quantity), the discharged NOx quantity is kept nearly constant (at the minimum) regardless of the quantity of the urea water.

With regard to a discharged NH₃ quantity, as shown with the long dashed short dashed line L2 in FIG. 4, in the region R1 where the urea water quantity is sufficiently low, the discharged NH₃ quantity is kept nearly constant (at the minimum) regardless of the quantity of the urea water. In the region R2 where the urea water quantity is larger than the optimum value of the urea water quantity (the optimum urea water quantity), as the quantity of urea water increases, the discharged NH₃ quantity (the redundant NH₃ quantity) increases.

As stated above, the optimum value of a urea water quantity (the optimum urea water quantity) is a turning point in the feature of the change of the exhaust gas characteristic relative to the change of the urea water quantity, namely in the feature of the change of the discharged NOx quantity and the discharged NH₃ quantity. Then the characteristics shown with the solid line L1 and the long dashed short dashed line L2 in FIG. 4 vary partially in response to the engine running state and the like. For example, as shown in FIG. 4, the basic trend is not changed but it sometimes happens that a positive or negative offset (level variation) is imposed on the urea water quantity (on the horizontal axis in the graph) or inclination or the like varies in a feature not shown in the figure. The way of variation is influenced by many parameters. Consequently, it is difficult to predict (estimate) the optimum value of the urea water quantity. In the present embodiment, the processing shown in FIG. 2 (the increment control) and the processing shown in FIG. 3 (the decrement control) are carried out alternately. As a result, even when the quantity of the urea water is either in the excessive region R2 or in the insufficient region R1, it is possible to bring the quantity of the urea water close to an optimum value by either the increment control or the decrement control.

That is, in the region R1 where the urea water quantity is small, it is determined that change exists at Step S23 in FIG. 3, namely the decrement control of the urea water quantity should not be applied and it is determined that change exists at Step S13, and thus the urea water quantity is increased as the processing of Step S12 until the urea water quantity comes to an optimum value. In the region R2 where the urea water quantity is large, it is determined that no change exists at Step S13, namely the increment control of the urea water quantity should not be applied and it is determined that no change exists at Step S23, and thus the urea water quantity Q is decreased as the processing of Step S22 until the quantity of urea water comes to an optimum value.

In this way, with the system according to the present embodiment, it is possible to cope with more situations flexibly, control the urea water quantity to an appropriate quantity with a high degree of accuracy, and purify an exhaust gas more appropriately.

As it has been explained above, with the quantity controller of an exhaust gas purifying agent and an exhaust gas purifying system according to the present embodiment, the following advantages can be obtained.

(1) The quantity of a purifying agent (NH₃) added into an exhaust gas to purify the exhaust gas by reacting with a specific component (NOx) in the exhaust gas is controlled. Such a quantity controller of an exhaust gas purifying agent is configured so as to have a program for carrying out either increment control (first quantity control, FIG. 2) for increasing the quantity of urea water (corresponds to the quantity of the exhaust gas purifying agent) sequentially at a predetermined rate α or decrement control (second quantity control, FIG. 3) for decreasing the quantity of the urea water sequentially at a predetermined rate β while monitoring, in NOx in the exhaust gas, the unpurified specific component quantity (the discharged NOx quantity) corresponding to the quantity not purified through the reaction with NH₃; and switching the quantity control from the currently adopted increment or decrement control to the other increment or decrement control in response to whether or not the feature of the change of the discharged NOx quantity comes to a predetermined feature (changed or not changed). More specifically, the quantity controller is configured so as to have a program (a quantity control means, FIGS. 2 and 3) for carrying out increment control while monitoring the discharged NOx quantity; when the feature of the change of the discharged NOx quantity comes to a predetermined feature (not changed) by the implementation of the increment control, stopping the increment control; then contrary to the increment control, carrying out decrement control to decrease sequentially the quantity of the urea water while monitoring the discharged NOx quantity; and when the feature of the change of the discharged NOx quantity comes to a predetermined feature (changed) by the implementation of the decrement control, stopping the decrement control. With such a configuration, it is possible to bring the quantity of the urea water close to an optimum value even though the quantity of the urea water is either in an excessive region or in an insufficient region.

(2) Further by adopting such a configuration, it is further possible to realize the longer service life and the simplification (downsizing) of an NH₃ catalyst 15, and others. Furthermore, it is possible to omit an NH₃ catalyst 15 in some applications, configurations, and others.

(3) Moreover with the above configuration, it is possible to control the quantity of urea water to an optimum value by the feature of the change of the discharged NOx quantity even when the discharged NOx quantity itself is not determined. Consequently, even when an optimum value of the quantity varies due to aging or the like, the accuracy of the quantity control (convergence to an optimum value) is kept at a high level.

(4) In FIGS. 2 and 3, after stopping currently adopted increment or decrement control, by carrying out the other increment or decrement control again, the increment control and the decrement control are alternately carried out repeatedly during the time when predetermined conditions are satisfied (for example, from engine start to engine stop). It is possible to control the quantity of urea water to an appropriate quantity consecutively with a high degree of accuracy while the increment control and the decrement control are carried out repeatedly.

(5) In FIGS. 2 and 3, the discharged NOx quantity is monitored with the actually measured values (values measured with the exhaust gas sensor 14 a), which improves the detection accuracy.

(6) The controller and the system are configured so as to be provided with a program for acquiring the quantity of urea water when the decrement control stops (an optimum quantity acquisition means, Step S24 in FIG. 3). By acquiring the optimum value of the quantity, it is possible to use the optimum value for, for example, data analysis, the correction of a purifying agent quantity characteristic, the failure diagnosis of an exhaust gas purifying system including the urea water feed valve 16 (the purifying agent feed valve), and others.

(7) The controller and the system are configured so as to be provided with a program for storing a urea water quantity acquired in the aforementioned program in a predetermined memory (an EEPROM, a backup RAM, or the like) that can retain data in a nonvolatile manner (Step S24 in FIG. 3). This facilitates, for example, data analysis by data accumulation and the like. Moreover, it is possible to retain data in a nonvolatile manner (the optimum value of the quantity) even after an engine stops (for example, an ignition switch is turned off) and electricity supply to the device (the ECU 40) stops; and carry out the data analysis, the correction, the failure diagnosis, and others also at the next engine start on the basis of the data at the engine stop.

(8) A urea SCR system is mounted on an automobile (a four-wheeled automobile in the present embodiment) on which a diesel engine is mounted. It is possible to try to improve fuel efficiency and reduce PM while allowing NOx to be generated during combustion, which realizes a cleaner diesel engine car with a high degree of exhaust gas purification.

(9) The exhaust gas purifying system is configured so as to be provided with a catalyst (an SCR catalyst 13) to accelerate specific exhaust gas purifying reaction (the reduction reaction (Expression 2)), a urea water feed valve 16 (a purifying agent feed valve) to add urea water (an additive) as the source for the production of NH₃ (a purifying agent) to purify an exhaust gas by carrying out the reduction reaction (Expression 2) with NOx in the exhaust gas on the SCR catalyst 13 to the exhaust gas on the upstream side of the SCR catalyst 13, and an ECU 40 for engine control. Then as stated above, it is configured so as to be provided with a program for carrying out either increment control or decrement control and switching the control from the currently adopted increment or decrement control to the other increment or decrement control in response to whether or not the feature of the change of the discharged NOx quantity comes to a predetermined feature (changed or not changed) while monitoring the discharged NOx quantity (the unpurified specific component quantity). By such an exhaust gas purifying system, it is possible to realize an exhaust gas purifying configuration with a higher degree of exhaust gas purification.

The above embodiments may be modified and carried out as follows.

In the above embodiments, a discharged NOx quantity is monitored on the basis of an actually measured value (an output from a sensor). The monitor of a discharged NOx quantity is not limited to this method however and a discharged NOx quantity (an unpurified specific component quantity) can be monitored by obtaining an occasional discharged NOx quantity by an arbitrary method. For example, a discharged NOx quantity may be estimated on the basis of the state of an exhaust gas (for example, an exhaust gas temperature detected with an exhaust gas temperature sensor or the like), a component (for example, an oxygen concentration detected with an oxygen concentration sensor), or the like.

It is also possible to bring the quantity of urea water close to an optimum value in a simple manner even by either the increment control (the processing shown in FIG. 2) or the decrement control (the processing shown in FIG. 3). For example, it is possible to bring the quantity of urea water close to an optimum value also by setting the initial value of a urea water quantity Q (a previous value) at a value in the region R1 of low quantity and exclusively carrying out only the processing shown in FIG. 2. In particular, when it is configured so as to judge whether or not a urea water quantity is sufficiently small and thereby it is determined that the urea water quantity is sufficiently small, it is effective to configure so as to set the initial value of a urea water quantity Q (a previous value) and carry out the processing shown in FIG. 2 as stated above. It is possible to bring the quantity of urea water close to an optimum value also by setting the initial value of a urea water quantity Q (a previous value) at a value in the region R2 of high quantity and exclusively carrying out only the processing shown in FIG. 3. In this case, when it is configured so as to judge whether or not a urea water quantity is sufficiently small and thereby it is determined that the urea water quantity is sufficiently small, it is effective to configure so as to set the initial value of a urea water quantity Q (a previous value) and carry out the processing shown in FIG. 3 as stated above.

It is also possible to use, instead of a discharged NOx quantity (an unpurified specific component quantity), an unreacted purifying agent quantity, in NH₃ (a purifying agent), corresponding to the quantity not consumed through the reaction with NOx (a specific component), namely a discharged NH₃ quantity measured with an exhaust gas sensor 14 a. Otherwise, it is also possible to install an NH₃ sensor on the downstream side of an NH₃ catalyst 15 and use, instead of the discharged NOx quantity, a released purifying agent quantity, in NH₃ (a purifying agent), corresponding to the quantity released in the air (usually on the downstream side of a muffler) on the downstream side of the site where NH₃ reacts with NOx (a specific component), namely a released NH₃ quantity measured with an NH₃ sensor on the downstream side of the NH₃ catalyst 15. As shown in FIG. 4, the discharged NH₃ quantity and the released NH₃ quantity show a trend opposite the case of a discharged NOx quantity with regard to a urea water quantity. As a result, when a discharged NH₃ quantity or a released NH₃ quantity is adopted, it is necessary to appropriately change the configuration of the above embodiment. For example in the case of FIG. 2, increment control stops when it is determined that change occurs at Step S13 and the increment control continues when it is determined that no change occurs at Step S13. Then in the case of FIG. 3, decrement control stops when it is determined that no change occurs at Step S23 and the decrement control continues when it is determined that change occurs at Step S23. With this configuration, it is possible to bring the quantity of urea water close to an optimum value. Then as stated above, the accuracy in the quantity control (convergence to an optimum value) is retained at a high level even when aging or the like exists.

Besides the above configurations, it is also possible to combine increment control with decrement control appropriately and obtain the relationship between a urea water quantity and a discharged NOx quantity (or a discharged NH₃ quantity or a released NH₃ quantity). For example, it is possible to obtain a trace or the like by plotting points on a graph. Then from the relationship thus obtained, it is possible to derive (detect) the optimum value of the urea water quantity. With such a configuration, it is possible to carry out data analysis and failure diagnosis or control the urea water quantity to an optimum value with a high degree of accuracy on the basis of the detected value (the optimum value).

It is possible to appropriately modify the kind and the system configuration of an exhaust gas source as a target of exhaust gas purification in response to the application and others. For example, when an automobile engine is the target of exhaust gas purification, the present invention is not limited to a compression ignition type diesel engine but applicable to a spark ignition type gasoline engine and the like and moreover is not limited to a reciprocating engine but applicable to a rotary engine and the like. Further, the present invention is applicable also to exhaust gas purification in an object other than an automobile, namely exhaust gas purification, for example, in a power plant, various kinds of factories, and others. With regard to the modification of a system configuration, when the discharged NH₃ quantity is sufficiently reduced in the configuration shown in FIG. 1 for example, it is possible to omit an NH₃ catalyst 15 or to take a similar measure.

Although the above embodiments and modified examples are explained on the assumption that various kinds of software (programs) are used, it is also possible to realize the similar functions with hardware such as an exclusively used circuit and the like.

Although a urea SCR (Selective Catalytic Reduction) system is demanded mainly at present, the present invention can be applied similarly to other applications when an exhaust gas is purified while a similar specific component is the object of purification and a similar purifying agent is used. 

1. A controller of an exhaust gas purifying agent to control a quantity of a purifying agent added into an exhaust gas to purify the exhaust gas by reacting with a specific component in the exhaust gas, the controller comprising: a quantity control means for carrying out a first quantity control for increasing or decreasing the quantity of the purifying agent sequentially at a predetermined rate while monitoring, in the specific component in the exhaust gas, a quantity of the unpurified specific component corresponding to a quantity not purified through a reaction with the purifying agent, and carrying out a second quantity control for decreasing or increasing the quantity of the purifying agent sequentially at a predetermined rate while monitoring the quantity of the unpurified specific component, wherein when a feature of the change of the unpurified specific component quantity comes to a predetermined feature by an implementation of the first quantity control, the first quantity control is stopped, and then, contrary to the first quantity control, the quantity control means carries out the second quantity control, and when the feature of the change of the unpurified specific component quantity comes to a predetermined feature by an implementation of the second quantity control, the second quantity control is stopped.
 2. A controller of an exhaust gas purifying agent according to claim 1, wherein either the first quantity control or the second quantity control is a control to increase the quantity of the purifying agent sequentially at a predetermined rate and to stop when the quantity of the unpurified specific component exceeds an allowable range and does not change any more even when the quantity is increased; and the other quantity control is the control to decrease the quantity of the purifying agent sequentially at a predetermined rate and to stop when the quantity of the unpurified specific component exceeds an allowable range and changes by decreasing the quantity.
 3. A controller of an exhaust gas purifying agent according to claim 1, wherein the quantity control means carries out the first quantity control and the second quantity control alternately repeatedly while predetermined conditions are satisfied by carrying out the first quantity control again after stopping the second quantity control.
 4. A controller of an exhaust gas purifying agent according to claim 1, wherein the quantity control means monitors the quantity of the unpurified specific component on the basis of an actually measured quantity of the unpurified specific component.
 5. A controller of an exhaust gas purifying agent according to claim 1, comprising an optimum quantity acquisition means for acquiring the quantity of the purifying agent when the second quantity control is stopped.
 6. A controller of an exhaust gas purifying agent according to claim 5, further comprising a means for storing the purifying agent quantity acquired by the optimum quantity acquisition means in a predetermined memory that can retain data in a nonvolatile manner.
 7. A controller of an exhaust gas purifying agent to control the quantity of a purifying agent added into an exhaust gas to purify the exhaust gas by reacting with a specific component in the exhaust gas, the controller comprising: a quantity control means for carrying out a first quantity control for increasing or decreasing the quantity of the purifying agent sequentially at a predetermined rate while monitoring, in the purifying agent, the quantity of the unreacted purifying agent corresponding to the quantity not consumed through the reaction with the specific component, and carrying out a second quantity control for decreasing or increasing the quantity of the purifying agent sequentially at a predetermined rate while monitoring the quantity of the unreacted purifying agent, wherein when a feature of the change of the unreacted purifying agent quantity comes to a predetermined feature by an implementation of the first quantity control, the first quantity control is stopped, and then, contrary to the first quantity control, the quantity control means carries out the second quantity control, and when the feature of the change in the unreacted purifying agent quantity comes to a predetermined feature by an implementation of the second quantity control, the second quantity control is stopped.
 8. A controller of an exhaust gas purifying agent according to claim 7, wherein either the first quantity control or the second quantity control is a control to decrease the quantity of the purifying agent sequentially at a predetermined rate, and to stop when the quantity of an unreacted purifying agent exceeds an allowable range and does not change any more even when the quantity is decreased, and the other quantity control is the control to increase the quantity of the purifying agent sequentially at a predetermined rate and to stop when the quantity of the unpurified specific component exceeds an allowable range and changes by the increase of the quantity.
 9. A controller of an exhaust gas purifying agent to control the quantity of a purifying agent added into an exhaust gas to purify the exhaust gas by reacting with a specific component in the exhaust gas, the controller comprising: a quantity control means for carrying out a first quantity control for increasing or decreasing the quantity of the purifying agent sequentially at a predetermined rate while monitoring, in the purifying agent, the quantity of the discharged purifying agent corresponding to the quantity discharged in air on the downstream where the purifying agent reacts with the specific component, and carrying out a second quantity control for decreasing or increasing the quantity of the purifying agent sequentially at a predetermined rate while monitoring the quantity of the unreacted purifying agent, wherein when a feature of the change of the unreacted purifying agent quantity comes to a predetermined feature by an implementation of the first quantity control, the first quantity control is stopped, and then, contrary to the first quantity control, the quantity control means carries out the second quantity control; and when a feature of the change of the unreacted purifying agent quantity comes to a predetermined feature by an implementation of the second quantity control, the second quantity control is stopped.
 10. A controller of an exhaust gas purifying agent to control the quantity of a purifying agent added into an exhaust gas to purify the exhaust gas by reacting with a specific component in the exhaust gas, the controller comprising: a practice control switching means for carrying out either a first quantity control for increasing the quantity of the purifying agent sequentially at a predetermined rate or a second quantity control for decreasing the quantity of the purifying agent sequentially at a predetermined rate while monitoring, in the specific component in the exhaust gas, the quantity of the unpurified specific component corresponding to the quantity not purified through a reaction with the purifying agent; and switching the quantity control from the currently adopted first or second quantity control to the other first or second quantity control in response to whether a feature of the change in the unpurified specific component quantity comes to a predetermined feature.
 11. A controller of an exhaust gas purifying agent to control the quantity of a purifying agent added into an exhaust gas to purify the exhaust gas by reacting with a specific component in the exhaust gas, the controller comprising: a practice control switching means for carrying out either a first quantity control for increasing the quantity of the purifying agent sequentially at a predetermined rate or a second quantity control for decreasing the quantity of the purifying agent sequentially at a predetermined rate while monitoring, in the purifying agent, the quantity of the unreacted purifying agent corresponding to the quantity not consumed through a reaction with the specific component; and switching the quantity control from the currently adopted first or second quantity control to the other first or second quantity control in response to whether a feature of the change in the unreacted purifying agent quantity comes to a predetermined feature.
 12. A controller of an exhaust gas purifying agent to control the quantity of a purifying agent added into an exhaust gas to purify the exhaust gas by reacting with a specific component in the exhaust gas, the controller comprising: a practice control switching means for carrying out either a first quantity control for increasing the quantity of the purifying agent sequentially at a predetermined rate or a second quantity control for decreasing the quantity of the purifying agent sequentially at a predetermined rate while monitoring, in the purifying agent, the quantity of the discharged purifying agent corresponding to the quantity discharged in air on the downstream where the purifying agent reacts with the specific component; and switching the quantity control from the currently adopted first or second quantity control to the other first or second quantity control in response to whether a feature of the change in the discharged purifying agent quantity comes to a predetermined feature.
 13. A controller of an exhaust gas purifying agent to control the quantity of a purifying agent added into an exhaust gas to purify the exhaust gas by reacting with a specific component in the exhaust gas, the controller comprising: a quantity control means for carrying out quantity control for increasing or decreasing the quantity of the purifying agent sequentially from a predetermined initial value at a predetermined rate while monitoring, in the specific component in the exhaust gas, the quantity of the unpurified specific component corresponding to the quantity not purified through the reaction with the purifying agent; and stopping the quantity control when a feature of the change in the unpurified specific component quantity comes to a predetermined feature by an implementation of the quantity control.
 14. A controller of an exhaust gas purifying agent to control the quantity of a purifying agent added into an exhaust gas to purify the exhaust gas by reacting with a specific component in the exhaust gas, the controller comprising: a quantity control means for carrying out a quantity control for increasing or decreasing the quantity of the purifying agent sequentially from a predetermined initial value at a predetermined rate while monitoring, in the purifying agent, the quantity of the unreacted purifying agent corresponding to the quantity not consumed through the reaction with the specific component; and stopping the quantity control when the feature of the change of the unpurified specific component quantity comes to a predetermined feature by an implementation of the quantity control.
 15. A controller of an exhaust gas purifying agent to control the quantity of a purifying agent added into an exhaust gas to purify the exhaust gas by reacting with a specific component in the exhaust gas, the controller comprising: a quantity control means for carrying out a quantity control for increasing or decreasing the quantity of the purifying agent sequentially from a predetermined initial value at a predetermined rate while monitoring, in the purifying agent, the quantity of the discharged purifying agent corresponding to the quantity discharged in air on the downstream where the purifying agent reacts with the specific component; and stopping the quantity control when the feature of the change of the unpurified specific component quantity comes to a predetermined feature by an implementation of the quantity control.
 16. A controller of an exhaust gas purifying agent to control the quantity of a purifying agent added into an exhaust gas to purify the exhaust gas by reacting with a specific component in the exhaust gas, the controller comprising: a means for detecting the quantity of the purifying agent with which the quantity of the unpurified specific component is minimum and also the quantity of the unreacted purifying agent is minimum by monitoring at least one of, in the specific component in the exhaust gas, the quantity of the unpurified specific component corresponding to the quantity not purified through a reaction with the purifying agent and, in the purifying agent, the quantity of the unreacted purifying agent corresponding to the quantity not consumed through the reaction with the specific component.
 17. A controller of an exhaust gas purifying agent to control the quantity of a purifying agent added into an exhaust gas to purify the exhaust gas by reacting with a specific component in the exhaust gas, the controller comprising: a means for detecting the quantity of the purifying agent with which the quantity of the unpurified specific component is minimum and also the quantity of the discharged purifying agent is minimum by monitoring at least one of, in the specific component in the exhaust gas, the quantity of the unpurified specific component corresponding to the quantity not purified through the reaction with the purifying agent and, in the purifying agent, the quantity of the discharged purifying agent corresponding to the quantity discharged in air on the downstream where the purifying agent reacts with the specific component.
 18. A quantity controller of an exhaust gas purifying agent according to claim 1, wherein the specific component is nitrogen oxide (NOx) and the purifying agent is ammonia (NH₃) produced by being added in the form of a urea aqueous solution and decomposing in an exhaust gas.
 19. An exhaust gas purifying system comprising: a catalyst to accelerate specific exhaust gas purifying reaction; a purifying agent feed valve to add a purifying agent to purify an exhaust gas by carrying out the exhaust gas purifying reaction with a specific component in the exhaust gas on the catalyst or an additive acting as the source of the purifying agent to the catalyst itself or the exhaust gas on the upstream side of the catalyst; and a practice control switching means for carrying out either first quantity control for increasing the quantity of the purifying agent sequentially at a predetermined rate or second quantity control for decreasing the quantity of the purifying agent sequentially at a predetermined rate while monitoring, in the specific component in the exhaust gas, the quantity of the unpurified specific component corresponding to the quantity not purified through the exhaust gas purifying reaction, and switching the quantity control from the currently adopted first or second quantity control to the other first or second quantity control in response to whether a feature of the change of the unpurified specific component quantity comes to a predetermined feature.
 20. An exhaust gas purifying system according to claim 19, wherein the specific component is nitrogen oxide (NOx), the additive is a urea aqueous solution, and the catalyst accelerates NOx reduction reaction to reduce NOx through ammonia (NH₃) produced by decomposing the urea aqueous solution as the exhaust gas purifying reaction. 